Climate-Driven Differences in Growth Performance of Cohabitant Fir and Birch in a Subalpine Forest in Dhorpatan Nepal
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Article Climate-Driven Differences in Growth Performance of Cohabitant Fir and Birch in a Subalpine Forest in Dhorpatan Nepal Raju Bista 1,* , Parveen K. Chhetri 1 , Jeremy S. Johnson 2 , Ashish Sinha 1 and Krishna B. Shrestha 3 1 Department of Earth Science and Geography, California State University Dominguez Hills, Carson, CA 90747, USA; [email protected] (P.K.C.); [email protected] (A.S.) 2 Department of Environmental Studies, Prescott College, Prescott, AZ 86301, USA; [email protected] 3 Department of Biological Sciences, University of Bergen, N 5020 Bergen, Norway; [email protected] * Correspondence: [email protected]; Tel.: +1-424-342-4852 Abstract: Himalayan Silver Fir (Abies spectabilis) and Himalayan Birch (Betula utilis) are tree species often found coexisting in sub-alpine forests of the Nepal Himalayas. To assess species-specific growth performances of these species, tree-ring samples were collected from the subalpine forest in the Dhorpatan Hunting Reserve, Nepal. Standard ring width chronologies of both species were correlated with climatic variables in both static and running windows. Differential and contrasting temporal responses of radial growth of these species to climate were found. Warmer and drier springs appeared to limit birch radial growth. Whereas radial growth of fir showed weakened climate sensitivity. Moving correlation analyses revealed divergent influences of spring climate on both fir and birch. Significant warming that occurred in the 1970s coincided with growth declines in Citation: Bista, R.; Chhetri, P.K.; birch and an increase in fir, as indicated by basal area increment. In summary, recent warming has Johnson, J.S.; Sinha, A.; Shrestha, K.B. been unfavorable for birch, and favorable to fir radial growth. Climate-Driven Differences in Growth Performance of Cohabitant Keywords: Abies spectabilis; basal area increment; Betula utilis; climate warming; growth-climate Fir and Birch in a Subalpine Forest in correlation; radial growth; tree-ring Dhorpatan Nepal. Forests 2021, 12, 1137. https://doi.org/10.3390/ f12091137 1. Introduction Academic Editor: Maciej Pach Environmental factors, dispersal, and biotic interactions are important fundamen- tal ecological filters shaping community structure and species co-occurrence [1]. Global Received: 10 July 2021 change drivers, particularly climate and land-use changes, induce individual or species Accepted: 20 August 2021 Published: 24 August 2021 level changes in plant phenology, physiological traits and habitat location [2]. Such changes influence species differently, thereby altering species coexistence mechanisms and bringing Publisher’s Note: MDPI stays neutral community level changes to local and regional scales [2]. Within a community, species with regard to jurisdictional claims in coexistence occurs through niche partitioning and differences in species specific key func- published maps and institutional affil- tional traits, such as phenology, depth of root systems, responses to varied environmental iations. gradients [2], trade-offs in their physiology, and wood structure and functioning [3]. There- fore, increased habitat heterogeneity supports the coexistence of multiple species with different requirements. Primary productivity among coexisting species varies with their life history and ecophysiological characteristics [4]. Vegetative as well as reproductive processes determine Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. species productivity and thus their competitive ability [5]. Because evolutionary trade-offs This article is an open access article in resource allocation between growth and reproduction regulate the productivity of a distributed under the terms and species, changes in environmental conditions, and the resulting selection pressure can result conditions of the Creative Commons in a species varying its key functional traits. Evolution and/or phenotypic plasticity can Attribution (CC BY) license (https:// enable a species to adapt to altered environmental settings [2]. Within the context of climatic creativecommons.org/licenses/by/ change, such species-specific traits and physiological responses to altered conditions help 4.0/). define the environmental niche of the species. Forests 2021, 12, 1137. https://doi.org/10.3390/f12091137 https://www.mdpi.com/journal/forests Forests 2021, 12, 1137 2 of 21 High elevation mountain areas are characterized by highly variable microsite con- ditions due to sharp changes in slope and aspect over a short distance. Studies have shown high-mountain plant communities are sensitive to climatic warming [6–8] and are experiencing rapid changes in composition and abundance. For example, Harsch et al. [9] reported 57% of worldwide treelines (including latitudinal) experiencing warming have advanced upslope and seen increases in recruitment. The Himalayas are experiencing greater warming than the global mean [7,8,10–12]. Forecasted warming is expected to bring dynamic changes to temperature sensitive subalpine plant communities [13,14]. Recent studies have shown increased tree recruitments and range shifts in the subalpine forests over the Nepal Himalayas at varying spatiotemporal intensity [8,10,15–20], with some instances of low recruitment and a lack of range shifts [10,21]. The recruitment performances and growth responses of trees at high elevation forests in the Nepal Himalayas are species- and site-specific [10,20,22] and impact how species will respond to climate change. While a warming climate can promote tree growth by lengthen- ing the growing season [23], the species specific traits offer varied fitness differences, and those trees with better fitness advantages are more likely to reproduce. For instance, in a resource (nutrient, water) limited habitat, warming induced moisture deficit can subject angiosperm trees to cavitation quickly compared to conifers with homoxylous woods [3]. By assessing how climate change impacts the growth and productivity of forests, we will have a better understanding of forest structure and community dynamics. Abies spectabilis and Betula utilis are two high elevation tree species from the Nepal Himalayas. Mixed forest stands of A. spectabilis and B. utilis are often found below the pure B. utilis belt below timberline in the central and western Himalayas [24]. Because angiosperms and conifers are quite different in their hydraulic safety margins [25], the contrasting ecophysiological trait differences allow these cohabiting species to respond dif- ferently to warming and droughts. While there have been varying site-and species-specific growth responses to climate, a more consistent finding is that A. spectabilis growth and treeline shift rate is higher than B. utilis [20]. Warm growing season temperature has often been found to favor the growth of A. spectabilis [10,11,13,17] in the eastern part of the Nepal Himalayas. However, radial growth of the same species from Rolwaling [12] and Mus- tang [19] was limited by spring moisture deficit. There is a general consensus that warming induced moisture stress during the spring disrupts growth in B. utilis [10,11,14,26,27]. Al- though mixed stands of A. spectabilis and B. utilis are often found in the subalpine zone, very few studies have compared both species [10,11,13,18,20], and their dynamics of cli- mate sensitivity have not been sufficiently addressed. We hypothesize that current climate trends have favored A. spectabilis compared to B. utilis in mixed stands. This study aims to look at the successional dynamics of A. spectabilis and B. utilis where they co-occur in the Nepal Himalayas. The long-term growth performances in relation to climate were assessed by (1) exploring the main climatic factors controlling radial growth, (2) examining the stability of the relationships, and (3) observing the temporal dynamics of species-specific productivity (rates of annual basal area increment) of both tree species. 2. Materials and Methods 2.1. Study Area The study site (28◦330 N, 83◦110 E, 3500–3600 m asl) lies in Rughakharka, in the eastern boundary of the Dhorpatan Hunting Reserve (hereafter DHR) in western Nepal (Figure1 ). Nearly 26.4%, 14.13% and 59.45% of DHR lie in Baglung, Myagdi, and Rukum District, respectively [28]. There is a wide range of topographic variations within DHR, and the climate and vegetation vary accordingly. Within DHR, different types of climate are found; ranging from sub-tropical climate at the lower elevation, to alpine at higher elevations [28]. The region receives monsoonal precipitation until late September. Daytime temperature in winter is very low and is often exacerbated by wind and clouds at high elevations. At treeline, snow cover persists until early April [29]. Forests 2021, 12, 1137 3 of 21 Figure 1. Land cover around the study area. The dots in the inset are the approximate position of Climatic Research Unit (CRU) grid cells. (Landcover data source: ICIMOD 2010). Supported by wide topo-climatic variation, subtropical to alpine vegetation are found in the region [28,29]. Common tree and shrub species found in the reserve are Abies spectabilis (silver fir), Pinus wallichiana (blue pine), Betula utilis (birch), Quercus semecarpifolia (oak), Rhododendron arboretum, and R. campanulatum (rhododendrons), Picea smithiana (spruce), Juniperus recurva (juniper), Tsuga dumosa (hemlock) and others [28,29]. Betula utilis is a dominant tree species within the treeline ecotone where Buki (Tussock sp.), grass (Carex sp.), R. anthropogon, and Cassiope fastigiata are the